1. Field of the Invention
This invention relates to ultrasonic transducers in general and more particularly to an ultrasonic transducer having an electronically adjustable mechanical lens permitting an increase in the depth of field for resolution perpendicular to the scanning direction of the transducer. The transducer may be either a linear array or a phased array.
2. Description of the Prior Art
The general characteristics of diagnostic ultrasound transducers are fairly well known in the prior art. Since the depth of field of present mechanical lenses is very limited, lenses have been proposed to image either close to or far from the scanned object, using two types of transducers for different applications. Alternatively, purely electronic solutions have been proposed. U.S. Pat. No. 4,371,805 discloses an ultrasonic transducer arrangement, the frequency of which is freely selectable within a certain range for the purpose of providing improved imaging conditions, particularly increased resolution in the production of images of a scanned space. This patent teaches an embodiment in which electronic focusing in the longitudinal direction as well in the transverse direction of the transducer should also be possible. The transducer uses a flat crystal without a mechanical lens. Consequently many acoustical lens elements are needed in the transverse direction. Thus, the major drawback of the transducer of this patent is the complex electronics required to make the design functional. British Pat. No. 1,514,050 is directed to an annular transducer arrangement with a fixed geometry rather than stepped electrodes. It uses a disc rather than a cylindrical lens and cannot be used in phased arrays.
U.S. Pat. No. 4,242,912 discloses a method for focusing an ultrasonic beam using time shifted pulsing of adjacent transducer elements, but not in conjunction with a prefocused mechanical lens.